Although wireless local area networks (WLANs) were initially designed for data communications, interests and demands for WiFi multimedia applications are growing rapidly. Video multicasting over IEEE 802.11 WLANs enables efficient distribution of live video or pre-recorded entertainment programs to many receivers simultaneously. However, digital video delivery requires high reliability, bounded delay and bandwidth efficiency. Wireless links are unreliable with time-varying and burst link errors. Specifically, in video multicast applications, different receivers of the same video may experience heterogeneous channel conditions. Receivers may also leave or join during the session so that the topology of network changes. In addition, there is no link layer retransmission and no link layer adaptation for multicasting in the current IEEE 802.11 standards. Erroneous packets are simply dropped. It appears to the application layer as a packet erasure channel. Packet loss can be detected by checking the sequence number field of the packet header. Therefore, it is important and a challenging task to support quality of services (QoS) for all the receivers of the multicast video in the desired serving area while efficiently utilizing the available WLAN resources.
In video multicast/broadcast over IP-based wireless networks, video data is encapsulated in UDP/IP packets and multicast/broadcast to the mobile devices over wireless networks. The IP-based wireless networks can be wireless local area networks (WLANs), cellular networks, wireless metropolitan area networks (WMANs) and wireless regional area networks (WRANs). When a mobile device moves from one cell to another, it is handed-over/handed-off from the base station (BS)/access point (AP) with which it is currently associated to another BS/AP. The two BSs/APs generally operate at different frequencies/channels. A number of packets are lost when the mobile device changes operating frequency to associate with the new BS/AP.
Typically, a broadcast signal is transmitted to all possible receivers simultaneously. A multicast signal is transmitted to a selected subset (one or more) of all possible receivers in a group simultaneously. As used herein multicast also includes broadcast. That is, a multicast signal may be transmitted to a selected subset of all possible receivers in a group where the selected subset may include the entire set of all possible receivers, i.e. the multicast group is all receivers.
In wireless systems, channel coding is used at the physical layer to protect packets against multipath lading and interference. However, channel coding within a packet cannot recover packet loss during handovers/handoffs.
One prior art method provides for transmission of duplicate data time-delayed/time-shifted from the original data (staggercasting) in an ATSC system to improve broadcast system robustness. When duplicate, time-staggered streams are sent, the system can tolerate signal loss up to the duration of the time shift between the two streams. Another prior art method provides a lower bit rate version of the original data (instead of the exact original data) is repetitively transmitted with a time delay. This approach reduces the bandwidth used by the redundant data. However, both of these prior art schemes send a composite signal and always send the signals whether or not there are any clients/receivers that want/need the data.
Yet another prior art method provided for the use of cross-packet forward error correction (FEC) codes to protect against synchronization loss in an ATSC system. FEC codes have also been used to recover lost packets in IP-based wireless networks. In general, an erroneous packet is discarded by the link layer. The FEC codes are applied across data packets at the transport and application layers and erasure decoding is used to recover the lost packets. However, the FEC parity packets are generally sent together with the data packet. During handoffs/handovers, long error bursts may occur. These long error bursts lead to the loss of data packets and parity packets exceeding the FEC capability, so that the lost data packets cannot be recovered.
There has been a great deal of research and theoretical analysis/simulations on various application layer forward error correction (FEC) and automatic repeat request (ARQ) algorithms to recover from packet loss and to improve transmission reliability in wireless networks. Another prior art method described an ACK-based hybrid ARQ algorithm for unicast video transmission and progressive video coding with FEC (MDFEC) for multicast video transmission over WLANs. Yet another prior art method provided receiver-driven FEC schemes for multicast in a wired Internet environment, in which FEC packets are delayed from the video packets. However, this method focused on how to optimize the performance of the heterogeneous receivers in a wired Internet environment.
The problem addressed and solved by the present invention is how to recover from random and burst packet loss, and achieve seamless handoffs to ensure high-quality video multicast/broadcast over IP-based wireless networks.